Zach Tellman started a clean room implementation of the event processing library lamina that is both cleaner and one and two orders of magnitude faster according to benchmarks. This speed comes at the cost of some changed semantics.

The reason some people are so excited about this patch is that currently the Clojure compiler nulls locals when it determines they will no longer be used, and this is a problem for debuggers, as when debugging you might want to know what the value was for that local, long after it has been cleared. This patch in Clojure 1.4 will allow you to disable this clearing in the compiler.